The present invention relates to solar cells and, more particularly, to photovoltaic solar cells.
Attempts to prepare molecular-based solar cells have generally employed molecules in conjunction with semiconducting films (often TiO2) or nanorods (such as CdSe). Conventional molecular-based cells employ several designs. For example, a monolayer of pigment or a thin film of pigment can be deposited on the semiconductor surface. As another example, U.S. Pat. No. 5,441,827 to Grätzel proposes a monolayer of pigment bound to a mesoporous semiconductor surface. The solar cells proposed in U.S. Pat. No. 5,441,827 to Grätzel generally require that the pigment be contained in a diffusive charge carrier, such as a gel or polymeric hole transport layer, to transport electrons.
In each of these designs, the absorption of light by the pigment can result in electron injection into the semiconductor. The hole residing on the pigment is then transferred (typically by a diffusive redox-active agent in a liquid) to the counterelectrode to complete the circuit. High solar-energy conversion efficiency typically requires absorption of most of the incident light across the solar spectrum, a high quantum yield of electron injection, a low quantum yield of charge recombination at the electrode surface, efficient electron transport in the semiconductor to the external circuit, and efficient transport of the hole to the counterelectrode.
However, in practice, conventional molecular-based cellular designs are limited in their efficiency. For example, a monolayer of photon-absorbing pigment typically absorbs only a tiny fraction of the incident light. A thin film can absorb much of the incident light, but the resulting excited-state energy may not reach the semiconductor surface, or the hole generated upon electron injection at the semiconductor may be trapped in the thin film and not reach the counterelectrode. In a monolayer of pigment bound to a mesoporous semiconductor surface, the mesoporous nature of the semiconductor material ensures high absorption of light. However, the mesoporous structure also presents many opportunities for electron-hole recombination. The efficiencies of these solar cells range from <1% for monolayer pigments to a few percent for thin film pigments and generally to a maximum of ˜10% for monolayers bound to a mesoporous semiconductor surface.
Quantum dots have also been used in solar cells. Quantum dots can provide intensive absorption, and the wavelength of absorption can be tuned by altering the size of the particles. However, the organization of the quantum dots in a superstructure to efficiently channel energy remains a challenge.